Showing papers by "Samir R. Das published in 2010"

Performance comparison of 3G and metro-scale WiFi for vehicular network access

Abstract: We perform a head-to-head comparison of the performance characteristics of a 3G network operated by a nation-wide provider and a metro-scale WiFi network operated by a commercial ISP, from the perspective of vehicular network access. Our experience shows that over a wide geographic region and under vehicular mobility, these networks exhibit very different throughput and coverage characteristics. WiFi has frequent disconnections even in a commercially operated, metro-scale deployment; but when connected, indeed delivers high throughputs even in a mobile scenario. The 3G network offers similar or lower throughputs in general, but provides excellent coverage and less throughput variability. The two network characteristics are often complementary. It is conceivable that these properties can be judiciously exploited for a hybrid network design where 3G data can be offloaded to WiFi for better performance and to reduce 3G network congestion and to lower costs.

Addressing deafness and hidden terminal problem in directional antenna based wireless multi-hop networks

Abstract: We address deafness and directional hidden terminal problem that occur when MAC protocols are designed for directional antenna based wireless multi-hop networks. Deafness occurs when the transmitter fails to communicate to its intended receiver, because the receiver's antenna is oriented in a different direction. The directional hidden terminal problem occurs when the transmitter fails to hear a prior RTS/CTS exchange between another pair of nodes and cause collision by initiating a transmission to the receiver of the ongoing communication. Though directional antennas offer better spatial reuse, these problems can have a serious impact on network performance. In this paper, we study various scenarios in which these problems can occur and design a MAC protocol that solves them comprehensively using only a single channel and single radio interface. Current solutions in literature either do not address these issues comprehensively or use more than one radio/channel to solve them. We evaluate our protocol using detailed simulation studies. Simulation results indicate that our protocol can effectively address deafness and directional hidden terminal problem and increase network performance.

Deconstructing Interference Relations in WiFi Networks

Abstract: Wireless interference is the major cause of degradation of capacity in 802.11 wireless networks. We present an approach to estimate the interference between nodes and links in a live wireless network by passive monitoring of wireless traffic. This does not require any controlled experiments, injection of probe traffic in the network, or even access to the network nodes. Our approach requires deploying multiple sniffers across the network to capture wireless traffic traces. These traces are then analyzed to infer the interference relations between nodes and links. We model the 802.11 MAC as a Hidden Markov Model (HMM), and use a machine learning approach to learn the state transition probabilities in this model using the observed trace. This coupled with an estimation of collision probabilities helps us to deduce the interference relationships. We show the effectiveness of this method against simpler heuristics, and also a profiling-based method that requires active measurements. Experimental results demonstrate that the proposed approach is significantly more accurate than heuristics and quite competitive with active measurements. We also validate the approach in a real WLAN environment.

Detecting selfish carrier-sense behavior in WiFi networks by passive monitoring

Abstract: With the advent of programmability in radios, it is becoming easier for wireless network nodes to cheat to obtain an unfair share of the bandwidth. In this work we study the widely used 802.11 protocol and present a solution to detect selfish carrier-sensing behavior where a node raises the CCA (clear channel assessment) threshold for carrier-sensing, or simply does not sense carrier (possibly randomly to avoid detection). Our approach is based on detecting any asymmetry in carrier-sense behavior between node pairs and finding multiple such witnesses to raise confidence. The approach is completely passive. It requires deploying multiple sniffers across the network to capture wireless traffic traces. These traces are then analyzed by using a machine learning approach to infer carrier-sense relationships between network nodes. Evaluations using a real testbed as well as ns2 simulation studies demonstrate excellent detection ability. The metric of selfishness used to estimate selfish behaviormatches closely with actual degree of selfishness observed.

Adaptive Spectrum Distribution in WLANs

Abstract: The IEEE 802.11 standard statically channelizes the wide spectrum into smaller channels of equal width, and classically, each access point is assigned one of these channels to operate on. While an intelligent channel assignment for interfering APs can provide better network performance than just any arbitrary channel assignment, still recent work shows that even far better performance can be achieved if the fixed 802.11 channels are not used and instead the channel width and central frequency for each AP is adapted, based upon the traffic load at each AP in the network. However, the existing work for dynamic distribution of spectrum amongst APs, provides non-overlapping channels to interfering APs, which can cause loss of spectral reuse opportunities. In this paper, we propose a new dynamic spectrum distribution technique in infrastructured wireless LANs, that exploits spectrum reuse opportunities and allows overlap between channels provided to interfering APs. Our technique achieves two goals: 1) max-min fairness amongst flows in the network and 2) high network throughput. Finally, we evaluate the performance of our technique, via simulations, and present its superiority when compared to single-channel 802.11-like DCF, classic fixed channelization and the state-of-the-art dynamic spectrum distribution technique for WLANs.

Inter-node communication method and system

Abstract: Disclosed is a data transmission system having a multi-node wireless network in a geographic area within which a plurality of nodes are positioned, having a plurality of directional antennas fixed to respective nodes of the plurality of nodes, with each node including at least two directional antennas to transmit and receive in respective sectors emanating from each respective node. When yaw of a node exceeds a threshold amount, revised routing information is provided to identify preferred directional antennas for use in future link transmissions.